It is well-known that temperature has a significant effect on the performance of pixilated semiconductor and semi-insulator materials used for photon counting in CT machines. In particular, CdTe, CdTe:Cl, CdZnTe, HgI2, and other semi-insulators exhibit poor carrier transport properties so that significant space charge can build within the material during use. This space charge affects the induced signal, resulting in tailing of the spectrum (i.e., spreading of the lower energy tail), spectral broadening, as characterized by FWHM measurements, and spectral shifts. The counter can become highly non-linear and, in extreme cases, even cease to function.
At low flux, space charge has very little effect. In certain medical applications a higher flux is used so that exposure time can be minimized. Inspection applications, for example in luggage scanning, require a high flux so that penetration can be maximized. In both cases sufficient photons must be collected to ensure statistical significance; it is inevitable that some level of space charge will build up during use.
The energy-discriminating systems currently being developed for medical and inspection applications use spectral integration to determine attenuation over specific energy ranges. Repeatable, non-linear effects can be removed mathematically from the data, provided that the response of the detector crystal does not vary with time. Controlling the temporal variation of space charge is thus critical to advance this technology.
The existing prior art, which deals mostly with scintillator/silicon devices, teaches that the temperature of detector material must be maintained within a narrow range to prevent significant changes in spectral response. Thermo-mechanical stability has also been identified as an important factor in CT applications.
U.S. Pat. No. 6,249,563 for X-RAY DETECTOR ARRAY MAINTAINED IN ISOTHERMAL CONDITION by Snyder et al. teaches the use of heat pipes to reduce temperature gradients within the detector arm of a CT scanner. Related U.S. Pat. No. 6,459,757 for X-RAY DETECTOR ARRAY WITH PHASE CHANGE MATERIAL HEAT SYSTEM by Joseph Lacey teaches the use of a phase change material and a sensor to measure and control the temperature within a detector array. However, this reference fails to control the temperature through the use of a feedback network.
U.S. Pat. No. 6,370,881 for X-RAY IMAGER COOLING DEVICE by Fyodor Maydanich teaches the use of a planar heat spreader disposed directly beneath an array of silicon detectors to maintain a nearly uniform temperature. The heat spreader is cooled with a thermoelectric device.
U.S. Pat. No. 6,931,092 for SYSTEM AND METHOD FOR THERMAL MANAGEMENT OF CT DETECTOR CIRCUITRY by Joshi et al. teaches the use of a heat sink to cool chips used for data acquisition within a CT machine. However, these chips are not located near the scintillator array and are more akin to the cooling schemes that are widely used in computers.
U.S. Pat. No. 7,135,687 for THERMOELECTRICALLY CONTROLLED X-RAY DETECTOR ARRAY STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH by Lacey et al. teaches the use of a thermoelectric system to control the temperature gradient across a large array of detectors. It cannot, however, adequately control a large array of detectors which will be subjected to very different thermal environments due to local conduction and convection effects. It also fails to address the need to reduce space charge at high flux levels, but seeks only to minimize spectral and special shifts with operating temperature.
Finally, U.S. Pat. No. 7,161,157 for SELF REGULATING DETECTOR RAIL HEATER FOR COMPUTED TOMOGRAPHY IMAGING SYSTEMS by Joseph Lacey teaches the use of PTC (Positive Temperature Coefficient) heaters within a detector array to maintain a uniform temperature. Since the current flowing through the heaters is a strong function of temperature, the devices become self-regulating. There is, however, a dependence on ambient temperature, air-flow and other variables. This scheme is therefore unlikely to provide the temperature stability needed to control variations in crystal response arising from the build-up of space charge.
In energy-discriminating CT systems, the performance of pixilated semiconductor and semi-insulator materials is improved when the temperature is raised above 40° C. due to increased carrier mobility.
None of the aforementioned inventions recognizes that detector crystals should be operated at these temperatures. The prior art is largely confined to scintillator/silicon technology; operating temperatures above 35° C. are not taught. Thus the electronics, which are typically reliable only at operating temperatures of less than 40° C., can operate in the same thermal environment as the detectors. This is no longer the case when crystal temperatures are increased to improve carrier mobility.
Further, a complication arises when the counting electronics must be located in proximity to the detector crystal. This requirement arises due to induced noise, low signal levels, and capacitance in the circuitry. It is the proximity of devices, with their conflicting thermal requirements, that has not previously been addressed.
Thus, it is desirable to provide independent means of controlling the temperature of semi-insulator, solid-state detector crystals within a narrow range while cooling their associated electronics so that performance, reliability and lifetime are maximized. Clearly a thermal solution is needed, such as this invention which can control and maintain an elevated temperature in detector crystals, either individually or in small groups, while cooling the associated electronics.